1. Field of the Invention
This invention relates to a method and an apparatus for static video telephone transmission/receiving, and more particularly to a method and an apparatus for a static video telephone transmission and receiving for modulating a static video signal captured by an image-capture system into a voice band signal, sending the modulated signal along a telephone line during a conversation, demodulating the static video signal received over the telephone line, and presenting an image on a display unit.
2. Description of the Related Arts
Although there has been a demand for a video telephone capable of transmitting voice and image data simultaneously over a telephone network, it is difficult to successively send and receive motion pictures which require a huge amount of data over the conventional voice grade telephone network. For this reason, a static video telephone for sending a static image to a speaker on the other end of the line during a conversation has now been put into practice.
With this static video telephone, during a normal voice conversation, it is possible to send images such as the face of a speaker, a picture or a drawing associated with contents of the conversation with a temporary interruption of the voice conversation.
Therefore, according to such a static video telephone, it is possible to realize transmission and receiving of image data which used to be impossible over the telephone line. The realization of a video telephone is advantageously facilitated because only one static image is transmitted at a time and the amount of data to be processed is limited.
Existing static video telephones capable of transmitting a static video image are introduced in a magazine entitled "TELEVISION TECHNICS & ELECTRONICS" VOL. 36 in 1988 pp. 19-35, and a block diagram of one of the video telephones is shown in FIG. 12 of the accompanying drawings.
As a technique for carrying out static video transmission set forth in the above, a phase-amplitude modulation technique is disclosed in U.S. Pat. No. 4,739,413. This technique is for sending monochromatic image data with its band width suppressed by the use of a multiple amplitude, two-phase modulation method.
In FIG. 12, a static video telephone 1 is composed of a telephone terminal 2 for connecting a non-illustrated external telephone set, a terminal 3 for telephone line input, a recording terminal 4 for connecting a tape recorder, a terminal 5 for image reproduction, telephone/reproduction changeover switch 6, and a key pad 7 for instructing video transmission, thereby connecting with external devices and the network.
Further, the static video telephone 1 is provided with a network circuit 8, a modulation-demodulation circuit 9 (hereinafter referred to as a "modem"), a demodulation timing generator 10, an analog/digital converter 11, a central processing unit 12 (hereinafter called a "CPU"), an image controller 13, an image memory 14, a digital/analog converter 15, a display 16, an analog/digital converter 17, and a television camera 18.
The operation of the conventional static video telephone will be explained with reference to FIG. 12.
In practice, a voice conversation input from the telephone set externally provided is transmitted from the telephone line terminal 3 via the external telephone terminal 2 and the network circuit 8. Meanwhile, a voice signal received from the remote end is transferred to the external telephone in reverse order to the transmission, namely, by way of the telephone line terminal 3, the network circuit 8, the external telephone terminal 2. and the non-illustrated external telephone set.
At the time of transmitting a video signal, the CPU 12 detects a transmission of video signal instruction issued by an operation of the key pad 7. The CPU 12 adds a control signal to image data memorized in the image memory 14 in accordance with a program stored in the CPU 12, and sends the image data with the information signal to the telephone line terminal 3 via the modem circuit 9 and the network circuit 8.
In the meantime, at the time of receiving a static video signal, the signal is transferred to the modem circuit 9 in the route reverse route to that of transmission. The video data from the partner in the conversation is demodulated by the modem circuit 9 with the use of a timing signal generated by the demodulation timing signal generator 10, and the demodulated video signal is then transferred to the image memory 14 into the analog-digital converter 11, the CPU 12 and the image controller 13, and once stored there can thereafter be shown on the display 16.
Upon completion of the transmission and receiving of the video signal, the network circuit 8 restores the video telephone 1 to a voice communication mode. The image captured by the television camera 18 during the conversation is digitized and stored in the image memory 14 by way of the image controller 13 and then the digitized data is used to produce an image on the display 16.
Referring to FIGS. 13, 14, 15 and 16, the structure of a frame format and the video signal transmission of a static video signal for use in a conventional static video telephone will be described hereunder.
The frame format of a static video signal for use in transmission and receiving of a static video signal is chiefly divided into two parts, that is, a control information block 19 and an image data signal 20 as shown in FIG. 15. The control information block 19 serves for demodulating an image data signal properly and consists of a frame synchronization signal 21, an amplitude calibration signal 22, and an information data signal 23 (hereinafter abbreviated as ID).
The frame synchronization signal 21 is that used for generating a demodulating timing signal as shown in FIG. 16A and also for switching between voice communication mode and video transmission/receiving mode. The amplitude calibration signal 22 is a signal for determining the level of automatic gain control (AGC) and thus the gradation of the video signal. The ID signal 23 is data providing a mutual identification capability.
Referring to FIGS. 13 and 14, the operation of the conventional demodulation timing generator 10 for use in the existing static video telephone as set forth in the above will be explained. FIG. 13 is a block diagram of the conventional demodulation timing generator 10, and FIG. 14 is a timing chart of the demodulation timing generator 10.
In FIG. 13, the conventional demodulation timing generator 10 comprises a zero-crossing detector 24 for detecting zero crossings of an input signal, a zero-crossing corrector 28 for producing an output of errors between a zero-crossing pulse signal 25 output from the zero-crossing detector 24 and a demodulation timing signal 26 as phase error data 27, and a demodulation timing generation block 29 for generating a demodulation timing signal 26 in response to the phase error data 27.
The operation of this demodulation timing generator 10 will be described hereinbelow.
Zero crossings at the trailing edge of a static video signal 30 input by way of the modem circuit 9 are detected by two flip-flops and a NOR gate in the zero-crossing detector 24, and are output as the zero crossing pulse signal 25. This zero-crossing pulse signal 25 is input to a correction period generating circuit 31 for determining a correction period.
At the correction period generating circuit 31, every time the zero-crossing pulse signal 25 is input, a signal 32 is output at a high logic level for a predetermined period equivalent to about half of 1747.8 MHz. During that time, a D-Type flipflop 33 of the zero crossing detector 28 detects whether the trailing edge of the demodulation timing signal 26 advances or retards compared with the zero-crossing pulse signal 25. When the demodulation timing signal 26 as shown in FIG. 14 is output from a programmable counter 34, for instance, the D-Type flipflop 33 outputs pulse signals 35 and 36 as shown in FIG. 14. During the high period of the signal 35, since the trailing edge of the demodulation timing signal 36 is delayed compared with the zero-crossing pulse signal 25, the programmable counter 34 is instructed so as to be advanced. However, during H period of the signal 36, since the trailing edge of the demodulation timing signal 26 advances compared with the zero-crossing pulse signal 25, the programmable counter 34 is instructed so as to be delayed. Both of these signals 35 and 36 are delivered to an AND gate 37 for generating an enable signal for an error detecting counter 41 and to an AND gate 38 for generating an enable signal for an error detecting counter 42, respectively.
An output signal 39 of the AND gate 37 represents the degree of delay of the demodulation timing signal 26 whereas an output signal 40 of the AND gate 38 represents the degree of advancement of the demodulation timing signal 26.
Counters 41, 42 in the next stage are clocked by a clock signal f.sub.CLKM which is obtained by dividing the reference clock 14.31818 MHz by a integral factor of M while the signals 39 and 40 are high. Values counted at these counters are transferred to a 2:1 selector 43. This selector 43 chooses a phase error data 44 when the signal 36 is low, but chooses a phase error data 45 when the signal 36 is high. The selected data 27 is then transmitted to a +8192 adder 46. The +8192 adder 46 adds 8192 to the phase error data 27 and outputs frequency dividing data 47 to the programmable counter 34. The programmable counter 34 divides the reference clock 14.31818 MHz in response to the frequency dividing data 47 and outputs it as the demodulation timing signal 26.
Thus, the conventional demodulation timing signal generator 10 can correct small impairments such as line shift. A maximum correction of the conventional generator 10 during one cycle of the input data is however limited to EQU (an amount of phase errors/M).times.2.
Since the conventional static video telephone has the structure being set forth in the above, if a frequency shift exceeds the maximum correcting amount of phase shift of the above, the demodulation timing signal 26 may not be corrected or may be unstable as represented by zero crossings a and b of FIG. 16B.